1. Field of the Invention
The invention generally relates to a float valve structure. The invention relates, for example, to a float valve structure in which fuel evaporative gas in a fuel tank of, for example, a motor vehicle flows to a canister and is then absorbed in an absorbent in the canister, or the fuel in the fuel tank is prevented from flowing to the canister when a fuel level rises.
2. Description of the Related Art
A motor vehicle or the like includes a fuel tank that stores fuel to be supplied to a combustion chamber of an engine. The fuel tank is provided with a ventilation system to permit air to appropriately flow in/out of the fuel tank in accordance with decreases/increases in the amount of fuel in the fuel tank. The ventilation system provides communication between the inside of the fuel tank and a canister. If the fuel tank is supplied with a greater amount of fuel than the tank capacity, the fuel that spilt out of the fuel tank is discharged to the canister. If the fuel that spilt out of the fuel tank is discharged to the canister, the canister becomes wet and unusable. To avoid such a problem, a full-tank control valve is provided at the upper portion of the fuel tank. When the fuel tank is filled up with fuel, the full-tank control valve blocks the ventilation system, thereby preventing the air and fuel from being discharged to the canister.
In addition to the full-tank control valve, the fuel tank is provided with a fuel-leak prevention valve that always contacts the atmosphere and suppresses pressure fluctuations in the fuel tank. The fuel-leak prevention valve is closed when the motor vehicle is tilted or overturned.
Generally, the full-tank control valve and the fuel-leak prevention valve are individually provided. However, a float valve serving as both the full-tank control valve and the fuel-leak prevention valve has already been proposed in order, for example, to reduce the cost.
Such a float valve is shown in FIG. 29. In a float valve 1, a valve support member 4 with a cylindrical body portion 4a and a float 5 are housed in an internal space 3 defined by a case 2. An upper opening 8a is formed in the upper portion of the float valve 1, and a lower opening 8b is formed in the lower portion of the float valve 1. The case 2 is housed in a fuel tank 6. The case 2 is arranged such that a flange 7, which is horizontally formed around the upper portion of the outer wall of the case 2, is fitted to the top wall surface of the fuel tank 6.
The float 5 moves upward when the fuel in the fuel tank 6 flows into the internal space 3 through the lower opening 8b. A rod-like first valve 9 having a small diameter is formed on the top surface of the float 5. A second valve 4b having a large diameter is fitted to the upper portion of the valve support member 4. The valve support member 4 is arranged above the float 5 so as to be vertically movable.
An upper-end opening 12 having a small diameter is formed in the center portion of the valve support member 4. A first valve seat 10 is formed in the inner surface of the center portion of the valve support member 4, at the position near the lower-end of the upper-end opening 12. The first valve 9 of the float 5 can contact the first valve seat 10.
A second valve seat 11 is formed at the end portion of the case 2, at the position near the lower-end of the upper opening 8a of the case 2. The float 5 and the valve support member 4 move upward together with each other, when fuel flows into the internal space 3 through the lower opening 8b, for example, when the fuel is supplied into the fuel tank 6. The first valve 9 of the float 5 contacts the first valve seat 10 of the valve support member 4, thereby blocking the upper-end opening 12. The second valve 4b contacts the second valve seat 11 formed at the position near the lower-end of the upper opening 8a, thereby blocking the upper opening 8a. Thus, the fuel is prevented from flowing to a canister (not shown) through a communication passage 13 formed above the upper opening 8a. 
When the valve is closed, the force with which the first valve 9 contacts the first valve seat 10 is considerably less than the force with which the second valve 4b contacts the second valve seat 11. Accordingly, if the pressure in the fuel tank 6 is reduced even by only a small amount, the float 5 moves downward due to its own weight. As a result, the first valve 9 moves away from the first valve seat 10, whereby communication is provided between the inside of the fuel tank 6 and the atmosphere through the upper-end opening 12.
As a result, the pressure in the fuel tank 6 is reduced, the second valve 4b immediately moves away from the second valve seat 11, and the upper opening 8a contacts the atmosphere. Accordingly, the pressure in the fuel tank 6 is reduced rapidly to the atmospheric pressure.
As described above, because the conventional float valve serves as both the full-tank control valve and the fuel-leak prevention valve, only one valve is required. Accordingly, the conventional float valve offers high cost performance and convenience, as described in Japanese Patent Application Publication No. JP-A-2004-257264.
Generally, a resin seal structure needs to offer high sealing performance. In addition, the production cost increases with increases in the number of portions where sealing needs to be provided. Therefore, the conventional float valve has a double seal structure in which two valve support members are used, as described above. In addition, in this seal structure, resin is used to form the first valve 9 and the first valve seat 10.
Accordingly, the float valve according to the related art can offer only limited sealing performance. Also, the double seal structure increases the production cost.
In order to address such problems, the seal structure including the first valve 9 having a small diameter and the first valve seat 10 may be omitted. However, such a structure causes another problem that the ease in re-opening the valve is reduced, for example, a greater force is required to cause the second valve 4b to move away from the second valve seat 11.